Optical scanning apparatus capable of reducing variations in shading and improving light usage

ABSTRACT

An optical scanning apparatus reduces shading and variations in light intensity and significantly increases light usage during an optical scanning process using a simple construction in which a laser beam from a light source is deflected by a light deflector having a reflective surface and is focused to a spot upon a scanning surface by a scanning lens to thereby perform optical scanning. The light source is arranged to produce a laser beam which includes both P-polarized light and S-polarized light. A direction of polarization of the light source is inclined in a plane perpendicular to the optical axis with respect to both the deflecting direction (the Y-axis direction) and a direction perpendicular to the deflecting direction (Z-axis direction). The laser beam impinges upon the reflective surface as polarized light which is between P-polarized light and S-polarized light, such that shading is minimized and variations in light intensity are significantly reduced and light usage is greatly increased during the optical scanning process.

This application is a Division of U.S. application Ser. No. 09/324,077,filed Jun. 1, 1999, U.S. Pat. No. 6,229,638, which is aContinuation-In-Part of U.S. patent application Ser. No. 09/031,410which was filed on Feb. 26, 1998, abandoned, the teachings of which areincorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a laser beam optical scanning devicefor writing data on an optical recording medium preferably for use in avariety of devices including a digital copying machine, a laser printer,an optical disk, and, in particular, to an optical scanning device thatis capable of reducing variations in shading and increase light usageduring a scanning process.

2. Description of the Related Art

An optical scanning device for scanning a scanning surface aligned withthe surface of an optical recording medium, such as a photosensitivematerial, is well known in relation to laser printers or the like. In anoptical scanning device, a usual optical arrangement is such that alaser beam from a laser beam source is deflected by a light deflector,such as a rotating polygon mirror, and applied to the scanning surfaceas a light spot by a scanning lens. Thus, the incident angles of thelaser beam on the reflective surface of the light deflector and on thescanning lens are caused to continuously vary during line scanning inthe main scanning direction.

Reflectance of the deflecting reflective surface and reflectance andtransmittance of the surface of the scanning lens vary in correspondencewith the respective incident angles, so that the intensity of the lightspot on the scanning surface generally fluctuates with an image heightin the main scanning direction, resulting in an unevenness in imagedensity in the line scanning or a deterioration in gradation. Thisphenomenon, which is referred to as “shading”, is serious when thedirection of polarization of the laser beam impinging upon thedeflecting reflective surface is parallel with or perpendicular to thedeflecting direction. Generally, the light intensity tends to be reducedor increased at either end in the main scanning direction with respectto the central image height in the main scanning direction.

To overcome this problem, a filter which has a transmittancedistribution has conventionally been used in such an optical system tocompensate for the variation in the intensity of light of the scanningline. However, there are limitations to the acceptable size and positionof the filter. Further, use of such a filter results in an increase incost of the optical scanning apparatus.

Japanese Patent Laid-Open No. 5-303049 proposed a construction in whicha ¼ wavelength plate is arranged in the optical path between the lightsource and the light deflector. Due to this construction, thepolarization of the beam impinging upon the light deflector is convertedto circularly polarized light, and reflectance of the reflective surfaceof the light deflector is kept substantially constant within thedeflecting region, whereby a reduction in shading is realized.

However, the above-described construction uses a ¼ wavelength plate,which is expensive, resulting in an increase in the production cost ofthe optical scanning device itself.

SUMMARY OF THE INVENTION

To overcome the problems described above, the preferred embodiments ofthe present invention provide an optical scanning device having a verysimple construction which significantly reduces shading, andsignificantly improves reflectance and light usage.

In accordance with a preferred embodiment of the present invention, anoptical scanning device includes a light source, which generates a laserbeam, a light deflector for deflecting the laser beam in a lightdeflection direction, and a scanning lens for focusing the deflectedlaser beam at a spot on a scanning surface to thereby perform opticalscanning, wherein the light source, the deflector, and the scanning lensare located along an optical axis, and the light source is tiltedrelative to the optical axis by an angle of about 17.5° to about 27.5°or by an angle of about 62.5° to 72.5°, and generates the laser beam,such that the laser beam, which is impinged on the reflective surface,is light polarized in a direction between a direction that is parallelto the light deflection direction and a direction that is perpendicularto the light deflection direction.

Thus, instead of using an arrangement of a light source which producesonly a P-polarized light or only an S-polarized light, the preferredembodiments of the present invention arrange a light source relative toan optical axis so as to produce a laser beam which includes acombination of P-polarized light and S-polarized light and minimizes theeffect of a plurality of disadvantageous conditions. As will bedescribed below, this arrangement of the light source and the resultinglaser beam including a combination of P-polarized light and S-polarizedlight significantly minimizes shading and reduces variations in lightintensity, while also maximizing light usage, during a scanningoperation.

The above-described preferred embodiment preferably includes an aperturelocated between the light source and the deflector. The aperturepreferably has a length that is larger than a width, where the lengthextends in a direction of a major axis of an ellipsoid shaped light beamoutput by the light source at a location of the aperture. Also, theaperture in this preferred embodiment preferably has a substantiallyrectangular shape or may have a substantially square shape.

In order to further maximize light usage, as described in more detailbelow, the corners of the aperture may be cut so as to define obliqueangles relative to sides of the aperture.

According to another preferred embodiment, an optical scanning apparatusincludes a light source generating a laser beam, a deflector having areflective surface arranged relative to the light source to deflect thelaser beam via the reflective surface in a light deflection direction, ascanning lens arranged relative to the deflector to focus the deflectedlaser beam at a spot on a scanning surface to thereby perform opticalscanning and an aperture located between the light source and thedeflector, the aperture having a substantially square shape, wherein thelight source, the deflector and the scanning lens are located along anoptical axis and the light source is tilted relative to the optical axisby an angle of about 45°, and generates the laser beam such that thelaser beam which is impinged on the reflective surface is lightpolarized in a direction between a direction that is parallel to thelight deflection direction and a direction that is perpendicular to thelight deflection direction.

As with the previously described preferred embodiment, this arrangementalso significantly minimizes shading and reduces variations in lightintensity, while maximizing light usage, during a scanning operation

The arrangement of the light source and the apertures described withrespect to the above preferred embodiments achieves an excellentcombination of elements and arrangement thereof which provides anextremely improved laser beam including P-polarized light andS-polarized light to produce the significant reduction in shading andvariation in light intensity, while maximizing light usage.

The above-mentioned light source may be an edge-emitting type laserdiode. In addition, the light source may have an array structureincluding a plurality of light emitting sections each disposed on acommon substrate and capable of independently effecting opticalmodulation.

These and other features, advantages and elements of the presentinvention will be apparent from the following detailed description ofthe preferred embodiments with reference to the accompanying drawings asdescribed below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view showing an optical scanning apparatus according toa preferred embodiment of the present invention;

FIG. 2(a) is a diagram illustrating a profile of a first surface of anoptical scanning lens according to a preferred embodiment of the presentinvention;

FIG. 2(b) is a diagram illustrating a profile of a second surface of anoptical scanning lens according to a preferred embodiment of the presentinvention;

FIG. 3(a) is a diagram illustrating a conventional arrangement of an endsurface emission type laser diode in which the active layer of the laserdiode coincides with a light deflecting direction;

FIG. 3(b) is a diagram illustrating another conventional end surfaceemission type laser diode in which the active layer of the laser diodeis perpendicular to a light deflecting direction;

FIG. 3(c) is a diagram illustrating an arrangement of an end surfaceemission type laser diode according to a conventional device in whichthe active layer of the laser diode is inclined at an angle of 45° withrespect to both a light deflecting direction;

FIG. 4a is a diagram illustrating a relationship between a lightpolarizing direction and a reflectance of a reflective surface of ascanning surface upon which the light from a light source is impingedfor various values of angle θ including a range of 62.5° to 72.5°according to a preferred embodiment of the present invention;

FIG. 4b is a diagram illustrating a relationship between a lightpolarizing direction and shading which is produced on the scanningsurface for values of angle θ shown in FIG. 4a;

FIG. 5a is a diagram illustrating a relationship between a lightpolarizing direction and a reflectance of a reflective surface of ascanning surface upon which the light from a light source is impingedfor various values of angle θ including a range of 17.5° to 27.5°according to another preferred embodiment of the present invention;

FIG. 5b is a diagram illustrating a relationship between a lightpolarizing direction and shading which is produced on the scanningsurface for values of angle θ shown in FIG. 5a;

FIG. 6a is a diagram showing a light usage amount when an aperturehaving a rectangular shape and an angle of tilt θ=90° according to acomparative example which was prepared for comparison to preferredembodiments of the present invention;

FIG. 6b is a diagram showing light usage amount when an aperture havinga rectangular shape and an angle of tilt θ=0° according to a comparativeexample which was prepared for comparison to preferred embodiments ofthe present invention;

FIGS. 6c and 6 d are diagrams showing light usage amount when anaperture having a rectangular shape and an angle of tilt θ=45° accordingto a comparative example which was prepared for comparison to preferredembodiments of the present invention;

FIG. 7a is a diagram showing light usage amount when an aperture havinga rectangular shape and an angle of tilt θ=22.5° according to apreferred embodiment of the present invention;

FIG. 7b is a diagram showing light usage amount when an aperture havinga rectangular shape and an angle of tilt θ=67.5° according to apreferred embodiment of the present invention;

FIG. 8a is a diagram showing light usage amount when an aperture havinga square shape and an angle of tilt θ=90° according to a comparativeexample which was prepared for comparison to preferred embodiments ofthe present invention;

FIG. 8b is a diagram showing light usage amount when an aperture havinga square shape and an angle of tilt θ=0° according to a comparativeexample which was prepared for comparison to preferred embodiments ofthe present invention;

FIG. 9 is a diagram showing light usage amount when an aperture having asquare shape including cut corner portions and an angle of tilt θ=45°according to a preferred embodiment of the present invention;

FIG. 10a is a diagram showing light usage amount when an aperture havinga rectangular shape including cut corner portions and an angle of tiltθ=22.5° according to a preferred embodiment of the present invention;

FIG. 10b is a diagram showing light usage amount when an aperture havinga rectangular shape including cut corner portions and an angle of tiltθ=67.5° according to a preferred embodiment of the present invention;

FIG. 11a is a graph of light distribution relative to an optical axisfor a conventional device showing the condition of side lobes beingcreated;

FIG. 11b is a graph of light distribution relative to an optical axisfor a preferred embodiment of the invention in which an apertureincludes cut corner portions as shown in FIG. 9 which eliminates theside lobes shown in FIG. 11a;

FIG. 12 is schematic perspective view showing a construction of aportion of a preferred embodiment of the present invention from thelight source to the reflective surface of a scanning surface upon whichlight from the light source is impinged;

FIG. 13 is a perspective view depicting a light source used in apreferred embodiment of the present invention, the light source having aplurality of laser diodes disposed in an array structure;

FIG. 14 is a diagram depicting how a light intensity distribution of alaser beam is defined by an aperture used in a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith reference to the attached drawings.

FIG. 1 is a diagram showing an optical scanning device in accordancewith a preferred embodiment of the present invention. In FIG. 1, apreferred optical scanning apparatus includes a light source 1 forgenerating a laser beam which is to be used in a scanning process aswill be described in detail below. A collimator lens 2 is arranged closeto the light source 1 for shaping a laser beam emitted from the lightsource 1 into a parallel beam. It should be noted that in one preferredembodiment of the present invention, the light source 1 and thecollimator 2 are integrally formed in a unitary body. Such an integralunit including the light source 1 and the collimator 2 allows for easierhandling, initial assembly and replacement.

An aperture 3 and a cylindrical lens 4 through which the laser beamcollimated by the collimator lens 2 is transmitted and which forms aline image elongated in a main scanning direction in the vicinity of areflective surface 5 a of a light deflector 5. The laser beamtransmitted through the cylindrical lens 4 impinges upon the lightdeflector 5 and is deflected so that the light impinges upon an opticalscanning lens 6 and is focused to a spot on a scanning surface 7 by theaction of the optical scanning lens 6 to perform optical scanning on thescanning surface 7 in the main scanning direction (the Y-axisdirection).

In FIG. 1, the angle α defined by the optical axis of the components ofthe optical system located before the deflector and the optical axis ofthe components of the optical system located after the deflector ispreferably about 60°. X1(Y) and X2(Y) indicate the profiles in the planeof deflection of the first and second surfaces, respectively, of thescanning lens 6 in the direction of the laser beam propagation (that is,the configuration as shown in FIG. 1). Both of the first and secondsurfaces of the scanning lens 6 as indicated by the profiles X1(Y) andX2(Y) have an aspheric profile, which can be expressed, for example, asfollows: assuming that the coordinate in the direction of the opticalaxis is X, the coordinate in the direction perpendicular to the opticalaxis is Y, the paraxial radius of curvature is R, and the higher-ordercoefficients are A, B, C, D, . . . , the following equation holds true:$X = {\frac{Y^{2}}{\left\{ {R + \frac{\sqrt{1 - {\left( {1 - K} \right)Y^{2}}}}{R^{2}}} \right\}} + {A*Y^{4}} + {B*Y^{6}} + {C*Y^{8}} + {D*Y^{10}} + \ldots}$

Further, in the preferred embodiment shown in FIG. 1, the opticalscanning lens 6 constitutes the scanning image formation lens itself,and establishes a conjugate relationship in a geometric-optic mannerbetween the position where the line image is formed and the scanningsurface 7 with respect to a sub-scanning direction (the Z-axis directionin FIG. 1). Further, it is shaped so as to satisfactorily compensate forthe curvature of field in the sub-scanning direction. Thus, the firstand second lens surfaces are “toric surfaces” as shown in FIGS. 2(a) and2(b).

The profiles of the surfaces of the optical scanning lens 6 with respectto the sub-scanning direction are indicated by X₁(Y) and X₂(Y), as shownin FIG. 1.

In the plane of deflection, the paraxial radius of curvature of thefirst and second lens surfaces are indicated by R1 and R2, and therefractive index of the lens material is indicated by N. Specifically,in this preferred embodiment, the scanning lens has the followingcharacteristics.

R₁=160.3, K₁=−58.38,

A₁=−9.22923E-07, B₁=3.65515E-10,

C₁=−8.34355E-14, D₁=1.113E-17,

R₂=−139.3, K₂=4.83,

A₂=−9.71348E-07, B₂=2.37E-10,

C₂=−8.06014E-14, D₂=2.65E-17.

The sub-scanning radius of curvature can be expressed by the followingequation.

rs(y)=r _(s)(0)+aY ₂ +bY ₄ +cY ₆ +dY ₈ +eY ₁₀ +fY ₁₂+

And, the sub-scanning radius of curvature preferably has the followingcharacteristics.

rs1(0)=−108.6, a₁=7.803E-02,

b₁=−3.15051E-04, c₁=8.16834E-07,

d₁=−1.10138E-09, e₁=7.352E-13,

f₁=−1.8802E-16

rs2(0)=−-15.09, a₂=−2.00512E-03,

b2=3.17274E-06, c₂=−4.04628E-09,

d2=5.72209E-12, e₂=−4.22019E-15,

f2=1.24827E-18.

Characteristics indicated in Table 1 below are also satisfied by thescanning lens used in the preferred embodiment.

TABLE 1 i R1 d1 N 0 33.2 1 160.3 13.5 1.51933 2 −139.3 128.3

FIGS. 3(a), 3(b) and 3(c) show conventional arrangements of lightsources.

The light source shown in FIGS. 3(a)-3(c) is an edge-emitting type laserdiode in which an active layer 1 c is placed between two cladding layers1 a and 1 b. The direction of polarization of the laser beam emittedfrom such an edge-emitting type laser diode is generally parallel to theactive layer 1 c.

In the first conventional arrangement shown in FIG. 3(a), the activelayer 1 c of the laser coincides with the light deflecting direction ofthe light deflector 5 (the Y-axis direction). In the second conventionalarrangement shown in FIG. 3(b), the active layer 1 c of the lasercoincides with a direction perpendicular to the light deflectingdirection of the light deflector 5 (the Y-axis direction).

In the third conventional arrangement shown in FIG. 3(c), the activelayer 1 c of the laser is inclined at a specific angle with respect toboth the light deflecting direction (the Y-axis direction) and thedirection perpendicular to the light deflecting direction (the Z-axisdirection). More specifically, the laser is rotated by an angle θ=45°around the optical axis. Numeral 1 d indicates the oscillation region inthe active layer 1 c.

The thick direction lines with arrows at either end thereof shown inFIGS. 3(a) through 3(c) indicate the respective polarization directionsof each of the arrangements shown therein. Polarization of the laserbeam in the conventional arrangements of FIGS. 3(a) and 3(b) arereferred to as P-polarized and S-polarized light, respectively, relativeto the reflective surface 5 a of the light deflector 5.

The reflective surface 5 a of the light deflector 5 preferably includesAl (aluminum) coated with SiO which is formed to have thickness of λ/2wherein λ=780 nm, and the optical scanning lens is preferably uncoated.Under these conditions, reflectance of the reflective surface 5 a of thelight deflector 5 and shading of the scanning surface 7 were measured.

FIGS. 4a, 4 b, 5 a and 5 b show reflectance and shading, respectively,with the light source being arranged as shown in FIGS. 3(a) through 3(c)at angles of θ=90° (FIG. 3a), θ=0° (FIG. 3b) and θ=45°. Also, shown inFIGS. 4a, 4 b, 5 a and 5 b are reflectance shading when the light sourceis arranged at angles of θ=17.5°, θ=22.5°, θ=27.5°, θ=45°, θ=62.5°,θ=67° and θ=72.5° according to preferred embodiments of the presentinvention.

As is apparent from FIGS. 4a, 4 b, 5 a and 5 b, when the P-polarizedlaser beam impinges upon the light deflector 5, reflectance decreases asthe angle of view decreases, and shading of the scanning surface 7decreases as the image height decreases. When the S-polarized laser beamimpinges upon the light deflector 5, reflectance decreases as the angleof view increases, and shading of the scanning surface 7 decreases asthe image height increases.

When the light source 1 is rotated by an angle θ=45° around the opticalaxis, as shown in FIG. 3(c), the direction of polarization of the beamimpinging upon the reflective surface 5 a of the light deflector 5 islocated directly between the P-polarized light and the S-polarizedlight. Although as is seen in viewing FIG. 3(c), the arrangement of thelight source emits a laser beam which is impinged on the reflectivesurface so as to include P-polarized light and S-polarized light (FIG.3(c)) and so that the laser beam impinges on the reflective surface at asubstantially perpendicular orientation relative to the reflectivesurface, this only addresses the shading and reflectance problem. Thatis, the angle of θ=45° was determined in the conventional device shownin FIG. 3(c) to be idea for providing good values for shading andreflectance.

However, it was not previously known that the θ=45° creates significantproblems with light usage. That is, although θ=45° produces goodreflectance and shading results, this angle causes problems with lightusage, as will be explained in more detail later.

Thus, the prior art, shown in FIGS. 3(a), 3(b) and 3(c) only recognizedthat angles of θ=0°, θ=45° and θ=90° could be used, and did notrecognize any problems with using a value of 45° for θ.

However, as is seen in FIGS. 4a, 4 b, 5 a and 5 b and also with respectto FIGS. 7a and 7 b described later, changing the angle θ to values offrom about 17.5 to 27.5 and from about 62.5 to about 72.5, each of theproblems of shading, reflectance and light usage are addressed in anextremely effective and successful manner. More specifically, by usingangles of θ equal to from about 17.5 to 27.5 and from about 62.5 toabout 72.5, reflectance and shading are very good as seen in FIGS. 4a, 4b, 5 a and 5 b, and also the light usage is excellent as seen in FIGS.7a and 7 b.

To see the differences in light usage, comparative examples shown inFIGS. 6a, 6 b, 6 c, and 6 d were prepared for comparison to the resultsachieved with preferred embodiments of the present invention shown inFIGS. 7a and 7 b.

As seen in FIG. 6a, a rectangular aperture is used and θ=90°. Lightusage is very good as seen by the fact that the periphery of theaperture is located completely within the beam spot. However, as seen inFIGS. 4a, 4 b, 5 a and 5 b, reflectance and shading are very poor whenθ=90°.

As seen in FIG. 6b, a rectangular aperture is used and θ=0°. Light usageis very good as seen by the fact that the periphery of the aperture islocated completely within the beam spot. However, as seen in FIGS. 4a, 4b, 5 a and 5 b, reflectance and shading are very poor when θ=0°.

As seen in FIGS. 6c and 6 d, a rectangular aperture is used and θ=45°.Although as seen in FIGS. 4a, 4 b, 5 a and 5 b, reflectance and shadingare good when θ=45°, light usage is very poor as seen by at least twolarge corner or peripheral areas of the aperture are located outside ofthe beam spot.

In order to maximize the quality of shading, reflectance and lightusage, the angle θ is changed to values of from about 17.5° to 27.5° andfrom about 62.5° to about 72.5° as described above. As seen in FIGS. 7aand 7 b, the light usage is very good when θ=22.5° and when θ=72.5°, andas seen in FIGS. 4a, 4 b, 5 a and 5 b, the shading and reflectance arealso very good when θ=22.5° and when θ=72.5°.

It should be noted that while the values of angle θ=22.5° and θ=72.5°are the most preferred for achieving an excellent combination of highquality shading, reflectance and light usage, other values of angle θfrom about 17.5 to 27.5 and from about 62.5 to about 72.5 can be usedand still achieve very high quality of each characteristic of shading,reflectance and light usage.

In the preferred embodiments shown in FIGS. 7a and 7 b, the shape of theaperture is preferably rectangular. A square shaped aperture may be usedin another preferred embodiment of the present invention shown in FIG.9. As seen in FIG. 9, when using a square shaped aperture, the angle θpreferably has a value of 45° so that the shading and reflectance arevery good as seen in FIGS. 4a, 4 b, 5 a and 5 b and so that the lightusage is also very good as seen in FIG. 9.

Compare the excellent results achieved in the preferred embodiment shownin FIG. 9 with those of the comparative examples shown in FIGS. 8a and 8b. Although light usage is good in FIGS. 8a and 8 b, where θ has a valueof 90 and 0 degrees, respectively, the shading and reflectance are verypoor when θ has a value of 90 and 0 as described above.

In another preferred embodiment shown in FIGS. 10a and 10 b, asubstantially rectangular aperture is used and θ has a value of about22.5° and about 67.5°, respectively. However, in order to provideexcellent quality of shading, reflectance and light usage, while alsoeliminating a side lobe problem shown in FIG. 11a, the aperturepreferably has cut corner portions. More specifically, at least two andpreferably four of the corner portions of the substantially rectangularaperture are cut so as to define oblique angles relative to the longerand shorter sides of the substantially rectangular aperture. As seen inFIGS. 10a and 10 b, the cut corner portions improve light usage evenfurther and as seen in FIG. 11b, eliminates the side lobe problem whichoccurs in FIG. 11a.

While in the above-described preferred embodiments a single laser beamis used, the light source 1 may be a monolithic semiconductor 20 inwhich a plurality of laser diodes 21, 22, and 23 are arranged in a rowin a single chip, for example, as shown in FIG. 13. By using such alight source, it is possible to write data on the scanning surface 7through a plurality of scanning lines at one time, thereby achieving anincrease in writing speed and resolution of written data.

As is the case in the above-described preferred embodiments, when thelight source comprises a semiconductor having a plurality of lightemission points, the direction of polarization of the emitted laser beamis inclined relative to the optical axis with respect to the deflectingdirection of the light deflector and the direction perpendicular to thedeflecting direction so that the direction of the laser beam impingingupon the reflective surface 5 a of the light deflector 5 may be betweenthe P-polarized light and the S-polarized light and include acombination thereof preferably at an angle θ having a value of about17.5° to 27.5° or about 62.5° to about 72.5°. As a result, it ispossible to make shading substantially constant within the scanningrange of the scanning surface. Thus, it is possible to obtain asatisfactory image having no variation in light intensity, while alsomaximizing light usage.

Generally, assuming that the deflecting direction of the reflectivesurface 5 a of the light deflector 5 is the main scanning direction (theY-axis direction) and that the direction perpendicular thereto is thesub-scanning direction (the Z-axis direction), the optical system fromthe light source 1 to the light deflector (the collimator lens 2, theaperture 3 and the cylindrical lens 4 in the case of FIG. 1) ispreferably arranged, as schematically shown in FIG. 12, so as to besymmetrical with respect to each of the directions. In order that thedirection of the laser beam emitted from the light source 1 may besubstantially between the P-polarized light and the S-polarized light asdescribed with reference to the above-described preferred embodiments,the arrangement and inclination angle of the light source 1 isdetermined and set before use of the optical scanning apparatus suchthat the polarization direction of the laser beam is preferably about17.5° to about 27.5° or about 62.5° to about 72.5° with respect to themain scanning direction and the sub-scanning direction.

When the light source 1 is an edge-emitting type laser diode, thedirection of polarization of the emitted laser beam is generallyparallel to the active layer, so that the light source 1 is preferablyinclined such that the active layer is substantially at an angle 17.5°to about 27.5° or about 62.5° to about 72.5° with respect to both themain scanning and sub-scanning directions.

When the light source is inclined as described above, the shape 1′ ofthe output laser beam is also inclined as shown in FIG. 14. This may bethe case when the edge emitting type laser diode, in particular, isused. In the case of the surface emitting type laser diode, since theoscillation region may be formed in any shape, it may be formedsubstantially as a circle, for example, so as not to affect the shape ofthe output laser beam even when the light source is inclined. However,generally, in the case of the edge-emitting type laser diode, theoscillation region is elongated in the direction of the active layer, sothat the emitted laser beam has an elliptical shape elongated in adirection perpendicular to the direction of the active layer. Thus, whenthe active layer of the edge-emitting type laser diode is inclined, thelaser beam has an inclined shape, and the light intensity distributionthereof is asymmetric relative to the directions respectivelycorresponding to the main scanning and sub-scanning directions.

To shape the asymmetric intensity distribution of the laser beam, theinner diameter of the aperture 3 is preferably made smaller than thesize of the collimated laser beam 1′ so that the beam spot on thescanning surface 7 becomes satisfactory, as shown in FIG. 14. The sizeand spot diameter of a laser beam preferably has a light intensity whichis substantially equal to 1/e² (which equals 0.135) of the maximum lightintensity. Accordingly, the asymmetric intensity distribution of thelaser beam may be sufficiently shaped when the inner diameter of theaperture 3 is included in an area which is set to be 1/e² of the centralintensity of the collimated laser beam 1′, as is the case of theaperture 3 shown in FIG. 16.

As described above, in accordance with the present invention, there isprovided an optical scanning device in which a laser beam from a lightsource is deflected by a light deflector having a reflective surface andis focused to a spot on a scanning surface by a scanning lens, whereinthe light source is inclined at an angle having a value of about 17.5°to about 27.5° or about 62.5° to about 72.5°0 with respect to both thedeflecting direction and a direction perpendicular to the deflectingdirection in a plane perpendicular to the optical axis, whereby thelaser beam from the light source impinges upon the reflective surface asa polarized light including P-polarized light and S-polarized light,thereby making it possible to provide maximum light usage whileminimizing shading and to reduce variations in the light intensity ofthe image.

When the light source is in an array structure having a plurality oflight emitting sections provided on the same substrate and which arearranged to independently effect light modulation, it is possible towrite with a plurality of scanning lines at one time.

When the light source is equipped with an angle adjusting unit foradjusting the above-mentioned inclination, a polarization inclinationangle of about 17.5° to about 27.5° or about 62.5° to about 72.5° can beachieved easily and reliably.

When an aperture is provided between the light source and the lightdeflector, and the inner diameter of the aperture is smaller than thediameter of an area determined to be 1/e² of the central intensity ofthe laser beam, the beam can be easily controlled, and a desired beamdiameter can be obtained on the scanning surface.

When the optical system from the light source to the aperture is formedas an integral light source unit, the angle of polarization can beeasily adjusted, and the handling of the apparatus is facilitated.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:
 1. An optical scanning apparatus, comprising: alight source generating a laser beam; a deflector having a reflectivesurface arranged relative to the light source to deflect the laser beamvia the reflective surface in a light deflection direction; and ascanning lens arranged relative to the deflector to focus the deflectedlaser beam at a spot on a scanning surface to thereby perform opticalscanning; wherein the light source, the deflector and the scanning lensare located along an optical axis and the light source is tiltedrelative to the optical axis by an angle of about 17.5° to about 27.5°and generates the laser beam such that the laser beam which is impingedon the reflective surface is light polarized in a direction between adirection that is parallel to the light deflection direction and adirection that is perpendicular to the light deflection direction. 2.The optical scanning apparatus according to claim 1, further comprisingan aperture located between the light source and the deflector, whereinthe aperture has a length that is larger than a width, where the lengthextends in a direction of a major axis of an ellipsoid shaped light beamoutput by the light source at a location of the aperture.
 3. The opticalscanning apparatus according to claim 1, further comprising an aperturelocated between the light source and the deflector, wherein the aperturehas a substantially rectangular shape.
 4. The optical scanning apparatusaccording to claim 3, wherein at least two corners of the aperture arecut to form oblique angles relative to sides of the substantiallyrectangular shaped aperture.
 5. The optical scanning apparatus accordingto claim 3, wherein at least two corners of the aperture are cut so asto have curved cut portions relative to sides of the substantiallyrectangular shaped aperture.
 6. The optical scanning apparatus accordingto claim 5, wherein at least two corners of the aperture are cut todefine linear cut portions relative to sides of the substantiallyrectangular shaped aperture.
 7. The optical scanning apparatus accordingto claim 3, wherein all four corners of the aperture are cut to formoblique angles relative to sides of the substantially rectangular shapedaperture.
 8. The optical scanning apparatus according to claim 1,wherein the light source is an edge emitting type light source.
 9. Animage forming apparatus comprising: a light source generating a laserbeam; a deflector having a reflective surface arranged relative to thelight source to deflect the laser beam via the reflective surface in alight deflection direction; and a scanning lens arranged relative to thedeflector to focus the deflected laser beam at a spot on a scanningsurface to thereby perform optical scanning; wherein the light source,the deflector and the scanning lens are located along an optical axisand the light source is tilted relative to the optical axis by an angleof about 17.5° to about 27.5° and generates the laser beam such that thelaser beam which is impinged on the reflective surface is lightpolarized in a direction between a direction that is parallel to thelight deflection direction and a direction that is perpendicular to thelight deflection direction.
 10. A method of manufacturing an opticalscanning apparatus comprising the steps of: providing a light sourcegenerating a laser beam; arranging a deflector having a reflectivesurface to deflect the laser beam via the reflective surface in a lightdeflection direction; arranging a scanning lens to focus the deflectedlaser beam at a spot on a scanning surface to thereby perform opticalscanning; and arranging an aperture between the light source and thedeflector, the aperture having a substantially square shape; wherein thelight source, the deflector and the scanning lens are located along anoptical axis and the light source is tilted relative to the optical axisby an angle of about 17.5° to about 27.5° and generates the laser beamsuch that the laser beam which is impinged on the reflective surface islight polarized in a direction between a direction that is parallel tothe light deflection direction and a direction that is perpendicular tothe light deflection direction.